671. Second Minimum Node In a Binary Tree

Given a non-empty special binary tree consisting of nodes with the non-negative value, where each node in this tree has exactly two orzero sub-node. If the node has two sub-nodes, then this node's value is the smaller value among its two sub-nodes.

Given such a binary tree, you need to output the second minimum value in the set made of all the nodes' value in the whole tree.

If no such second minimum value exists, output -1 instead.

class Solution {

    public int findSecondMinimumValue(TreeNode root) {

        if (root == null) {

            return -1;

        }

        Set<Integer> set = new TreeSet<>();//treeset把元素存储在红黑树中，所以这个set是有序的，从小到大

        dfs(root, set);

        int count = 0;

        Iterator iterator = set.iterator();

        while (iterator.hasNext()) {

            count++;

            int result = iterator.next();

            if (count == 2) {

               return result;

            }

        }

        return -1;

    }

    private void dfs(TreeNode root, Set<Integer> set) {

        if (root == null) {

            return;

        }

        set.add(root.val);

        dfs(root.left, set);

        dfs(root.right, set); 

    }

}